Electrical utility workers, telephone linemen, appliance installation teams, and other types of workers often find themselves working among or near electrical conductors that could potentially carry large dangerous voltages. Electrical utility workers and telephone linemen, for example, often ply their trades near high voltage power lines. Even in situations where these individuals are not working directly in the vicinity of power lines, they nevertheless find themselves working on or with other metal items that can conduct potentially dangerous voltages under certain circumstances. These include electrical and telephone junction boxes, power distribution stations, circuit breaker panels, and even the metal exterior skin of house trailers and other metal buildings. It is extremely important that such workmen be able to determine whether these conductors and conducting surfaces carry potentially dangerous voltages before coming into contact with the conductor.
One method of determining whether a conductor is carrying a voltage is simply to measure the voltage directly with a volt meter connected between the conductor and ground. While this method indeed gives an accurate measurement of voltages present on the conductor, it is nevertheless dangerous and highly undesirable, particularly when measuring voltages greater than about 150 volts RMS. This is because direct contact is required between the volt meter probe and the conductor. Further, in many situations, such as with electrical power lines, the conductor is covered with an insulator so that direct contact with a volt meter probe is not practical.
Attempts to design voltage detectors that read the presence of a voltage on a conductor without actually contacting the conductor have been made. U.S. Pat. No. 5,414,344 of Chinn, for example, discloses a non-contact apparatus for sensing energization of high voltage signal lines. The device disclosed in this patent senses the presence of an electrical field with a metal antenna that can be varied in length to adjust the sensitivity of the device. The antenna is placed a distance from a suspected high voltage conductor, such as a power line, and, if a substantial E-field is detected, appropriate LED's can be lit to indicate that the monitored conductor is active. In addition, digital signals are produced so that breaker motors can be activated to engage or disengage a power line breaker upon the occurrence of certain criteria. While this device represents an improvement over direct contact measurements, it nevertheless has numerous inherent problems and shortcomings. For example, as disclosed in the specification of Chinn, this device is useful only for detecting very high voltage of approximately 2,000 volts and higher on power distribution lines. The device is not intended for use by workmen but rather is intended for permanent installation in a power substation for remotely monitoring electrical distribution lines and controlling substation relays. The Chinn device is not concerned with detecting relatively low voltages that might nevertheless be dangerous if contacted by a human. In addition, no filter means is provided for filtering out the presence of known but non-dangerous voltages that may be present on the conductor.
U.S. Pat. No. 5,325,047 of Kempton discloses a voltage monitoring apparatus for monitoring cathodic protection voltage on an underground pipeline. The device disclosed in this patent is a very simple analog device that requires direct connection to a conductor and to ground for purposes of measuring the presence of a protective voltage. Obviously, such direct connection is undesirable for use by electric utility workers and linemen.
Finally, U.S. Pat. No. 4,818,945 of Bossard discloses a non-contacting volt meter comprising a multi-layer capacitor that forms an arrangement of three coupled capacitors. This arrangement, according to Bossard, exhibits low time constants that permit detection of voltages in the megahertz range. Such detection is useful in electrostatic copying machines.
The devices of these patents fails to meet the needs of workers wishing to detect the presence of dangerous voltage potentials. Further, none of them suggests or even mentions an issue of particular interest to electric utility workers and telephone linemen; that is, the ability to detect potentially dangerous voltages on a conductor while ignoring the presence of known benign voltages. For example, potentially dangerous voltage signals usually are alternating current signals having a primary frequency of 60 hertz and harmonics up to about 180 hertz. Voltage signals above about 200 hertz are usually not dangerous but nevertheless can be induced in conductors by radio transmitter signals, microwave signals, and the like. These higher frequency signals are not potentially dangerous to the workmen but nevertheless can affect voltage detectors corrupting their measurements or producing a false alarm. Another benign signal commonly present among telephone circuits is the telephone ring signal. By far, the predominant ringing signal used in the United States telecommunications industry is a 20 hertz sine wave signal at about 85 to 105 volts RMS. While such a signal is common, it is nevertheless not considered dangerous by linemen and utility workers. However, detection of such a signal by a voltage detector could corrupt a measurement or indicate the presence of a dangerous condition when, in fact, no dangerous condition exists.
Accordingly, there exists a need for a method and apparatus for detecting and indicating to a worker the presence on electrical conductors of potentially dangerous voltage potentials. Such a method and apparatus should effectively filter out the presence of electrical signals having frequencies above about 200 hertz to filter out stray signals caused by, for example, transmitters. Furthermore, such a detector should accurately and effectively filter out signals produced at about 20 hertz that correspond to telephone ringing signals so that these signals will not corrupt measurements or produce a false alarm. Finally, such a method and apparatus should be configured in a small handheld device that can be carried by utility workers and linemen and should have a fully insulated probe that can be positioned adjacent suspected conductors without actually contacting the conductors to measure the presence of a voltage on the conductor. The apparatus should be convenient, reliable, accurate, and should be economical and feasible to produce in large quantities. It is to the provision of such a method and apparatus that the present invention is primarily directed.